I've been using an Optima 6 volt battery in Gav's GP for the last 2 years and believe they are a great option for anyone wanting to keep their jeep 6 volt. If you get an additional old school looking battery box to conceal the optima then they look great under the bonnet too. Here are some photos of the setup.

Good tip Aaron and looks great installed.
Our late and great member Charlie Anderson, had a 12 volt Optima lying side on under the passenger seat of his Jeep, powering goodness knows what, but knowing Charlie, it would work well!
The Optima is expensive, but is worth it for special situations.